3D Printing and processing of miniaturized transducers with near-pristine piezoelectric ceramics for localized cavitation

The performance of ultrasonic transducers is largely determined by the piezoelectric properties and geometries of their active elements. Due to the brittle nature of piezoceramics, existing processing tools for piezoelectric elements only achieve simple geometries, including flat disks, cylinders, cubes and rings. While advances in additive manufacturing give rise to free-form fabrication of piezoceramics, the resultant transducers suffer from high porosity, weak piezoelectric responses, and limited geometrical flexibility. We introduce optimized piezoceramic printing and processing strategies to produce highly responsive piezoelectric microtransducers that operate at ultrasonic frequencies. The 3D printed dense piezoelectric elements achieve high piezoelectric coefficients and complex architectures. The resulting piezoelectric charge constant, d33, and coupling factor, kt, of the 3D printed piezoceramic reach 583 pC/N and 0.57, approaching the properties of pristine ceramics. The integrated printing of transducer packaging materials and 3D printed piezoceramics with microarchitectures create opportunities for miniaturized piezoelectric ultrasound transducers capable of acoustic focusing and localized cavitation within millimeter-sized channels, leading to miniaturized ultrasonic devices that enable a wide range of biomedical applications.


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Supplementary Information 1 to 8 33 Supplementary Fig. S1 to S13 34 Table S1  35 References 36 SI. 1 Fabrication resolution of the 3D piezoelectric nanocomposites 37 During the curing process, the resolution of the custom PμSL fabrication system in x-y plane 38 (20´20 µm) is determined by the pixel size of the projector. The resolution along z-axis is 39 determined by the curing depth of colloid. The curing depth of the solid layer is given as 1 : 40 where, a is the resin absorption coefficient, E and Ec are the actual and critical exposure, 42 respectively. In our fabrication process, the layer thickness was set as 15 µm, which is smaller than 43 half of the cure depth to ensure tight bonding between two consecutive layer 2 . This optimized cure 44 depth for each layer was modulated by tuning the exposure time 3 . A reductive lens was utilized to 45 increase the fabrication resolution of the printed structure 3 . The range of achievable 3-3 vibration 46 mode frequency of a 3D printed thin-film sample is determined by the range of thickness that can 47 be achieved, which is around 10.8 µm. This corresponds to the highest achievable frequency on 48 the order of 171 MHz. Higher frequency can be potentially achieved by reducing the minimal layer 49 thickness of the printing apparatus (employing Z-stage with higher resolution). 50 51 SI. 2 Element shrinkage during sintering and its effects on the structural 52 integrity 53 To evaluate the effects of shrinkage on structural integrity, we measured three types of geometrical 54 parameters before and after sintering, including length change (characterized on flat samples), 55 curvature deviation (measured on concave samples) and strut thickness deviations. Here, 20 56 rectangular samples (3´2.6´0.2 mm) were fabricated and the dimension change before and after 57 sintering was measured, as shown in Fig. S4a thickness of the sintered struts is less than 1.58% thicker than the as-designed value (300 µm) with 67 a standard deviation of 6.6 µm, indicating that the structural integrity was well kept after sintering. 68

Measurement of acoustic impedance 70
The schematic of the setup that was used to measure sound speeds is shown in Fig. S5a. The sound 71 speed in the material Vmaterial calculated using the phase change Df of the received signal after inserting the material in between the two standard transducers (Fig. S5c) via: 73 where, t indicates the thickness of the material to be tested, -$"%& is the sound speed in 75 water. The acoustic impedance is calculated by 4 : 76 where, r is the density of the material.

Measurement of attenuation coefficients 79
The transmission ratio, tp,between the transmitted wave acoustic pressure and the incident wave 80 acoustic pressure on the interface 5 can be represented by: 81 where the Z1 and Z2 indicates the acoustic impedance of the two materials. Thus, the acoustic 83 pressure p in the material near the interface I and interface II ( The backing material is the Fe magnetic particle-loaded Flexible resin (Formlab, USA). Flexible 121 is the mixture of acrylated oligomers and acrylated monomers. By tuning the Fe magnetic particle 122 loading in the backing layer from 3 wt% to 20 wt%, the attenuation coefficient of the backing 123 materials can range from -4.3 dB/mm to -10 dB/mm for 10-MHz sound waves damping. The Fe 124 magnetic particle loading can increase sound dissipation by increase the thermal loss caused by 125 the friction of particles 8 . The acoustic impedance of the backing material ranged from 2.6 Mrayl 126 to 8 Mrayl, making it suitable for many transducer applications mentioned in the main text. 127

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The design, material selection, and assembly of the matching layers depend on the intended 129 applications and the function of the transducers. Using the transducers with 3-3 working modes as 130 an example, which is also a commonly used ultrasonic transducer type for imaging and 131 nondestructive testing, the matching layer is designed based on the shape of the aperture of the 132 transducer. The material selection is determined by the sound propagation medium, as we 133 mentioned in the manuscript. The thickness of matching layer was controlled to be less than one-134 fourth of the ultrasound wavelength to minimize the acoustic attenuation 9 . For our focused 135 miniatured transducer, we 3D printed the matching layer to fit the size of the sintered piezo element 136 and glue the matching layer with the piezo part using the same photocurable material used for the matching layer. For the transducer with microarchitectures, such as the lattice transducers, we 138 designed the matching layer as flat membranes and glue it onto the surface that receives sound 139 wave. 140 SI. 6 Benchmark the energy output capability and sound beam lateral 141 resolution of our 3D-printed transducer with the state-of-the-art 142 We benchmark our 3D-printed transducer with reported micro transducers in Table S1. Normalized 143 &#6 is defined as the negative acoustic pressure generated by the element per unit voltage input 144 per surface area and used to describe the energy output capability. For the studies that use acoustic 145 intensity to characterize the performance of the transducers, we convert the spatial-peak-pulse-146 average intensity Isppa into the root-mean-square acoustic pressure &#6 via 10 147 &#6 = ? 6..$ 7 6.1 148 where ro is the density of sound propagation medium and c is the sound velocity in the medium.

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As shown in the The lateral resolution of ultrasonic transducers is determined by the geometries and driving 153 frequencies of piezo elements 16 . We simulated the acoustic intensity field of our transducer with 154 COMSOL by inputting transducer aperture geometry and working frequency (Fig. S7a)